Borosilicate glasses include many groups of glasses having known desirable characteristics such as high thermal and chemical stability and high mechanical strength.
The borosilicate glasses can be processed without difficulty by machine in accordance with conventional technologies from the melt flux.
Corresponding to the special application purpose of the glasses, individual ones of the above-mentioned or additional characteristics have been developed further by special glass compositions. The characteristics include, for example, the electrical conductivity, the fuse characteristics, the transmission, the absorption (including X-ray absorption) or the thermal tempering or the chemical tempering.
However, from the state of the art, it is apparent that a requirement for glasses having improved characteristics exists which have as many as possible of the desired physical-chemical characteristics simultaneously.
In Table 1, borosilicate glasses of high thermal and chemical stability with slight thermal expansion are listed with their compositions and selected characteristics.
It becomes clear that the borosilicate glasses in the area of thermal expansion (.alpha..sub.20/300 approximately 4 to 5.times.10.sup.-6 K.sup.-1) have no optimal resistance with respect to alkali solutions. It is also clear that the reference glass "borosilicate glass 3.3" (DURAN.TM.) of the alkali-resistant class 2 likewise has no optimal resistance with respect to alkali solutions.
According to DIN 52 322, the glasses are subdivided into three LBK classes in dependence upon their resistance to the attack of alkali solutions The classes are as follows:
______________________________________ Extent of Attack by LBK-Class Weight Loss mg/dm.sup.2 Alkali Solutions ______________________________________ 1 up to 75 slight attack 2 greater than 75 and up to 175 moderate attack 3 greater than 175 intense attack ______________________________________
The glasses of Class 1 of alkali resistance should accordingly have a removal of maximally 75 mg/dm.sup.2.
There are many borosilicate glasses known in the system SiO.sub.2 --B.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --M.sub.2 O--MO (wherein M.sub.2 O=alkali oxides and MO=earth alkali oxides and ZnO) which have an alkali resistance with a removal of approximately 100 mg/dm.sup.2 such as shown in Table 2, Example 1.
This resistance against alkali-solution attack is inadequate for the highest requirements and can be improved in a manner known per se by stabilizing admixtures and especially by the admixture of ZrO.sub.2. The positive effect of ZrO.sub.2 admixtures is described in the technical literature and in patent publications. In this connection, reference can be made, for example, to the text of Thiene entitled "Glas", (1938), Volume 2 pages 634 and 635; Scholze, entitled "Glas", Springer Verlag, (1988), page 321, and in patent publications such as British Patent 791,374; U.S. Pat. No. 4,259,118, German Patent 3,722,130 and German patent publication DD 301,821 A7.
In Table 2, melt examples are listed from German patent publication DD 301,821 A7 which show that the alkali attack can be reduced with admixtures of approximately 1 to 2 percent by weight of ZrO.sub.2 so that in some cases LBK-Class 1 is reached (see Table 2, Examples 4 and 5).
The samples of the glass of Example 5 of Table 2 were melted in three separate test series and each of these three samples was then measured. The resistance to alkali solution obtained with the removals 70, 74 and 79 mg/dm.sup.2 is therefore not to be included in LBK-Class 1. Increasing the ZrO.sub.2 admixture to further improve and therefore stabilize the resistance to alkali solution serves no purpose because melt and cost problems occur, that is, the crystallization stability of the glass is lost.
From the example of the melts of Table 2, Example 5, it becomes clear that the removal must be dropped to below approximately 70 mg/dm.sup.2 for a stable adjustment of the class 1 glasses in order to provide a "stability reserve". Such a reserve is even then purposeful when a glass composition provides removal values during reproduction meltings which scatter less and lie, for example, in the range of 70 to 73 mg/dm.sup.2.
In practice, small changes of the determined composition must be made, however, in order to adapt the glass, for example, to the corresponding melt process. For this reason, a synthesis-produced change of the removal value is often unavoidable. The known borosilicate glasses in the system (SiO.sub.2 &gt;70 percent by weight; B.sub.2 O.sub.3 &lt;15 percent by weight; Al.sub.2 O.sub.3 &lt;10 percent by weight; M.sub.2 O&lt;10 percent by weight; MO&lt;10 percent by weight; and, ZrO.sub.2 &lt;5 percent by weight) have either the disadvantage that they do not realize the ratio SiO.sub.2 :B.sub.2 O.sub.3 &gt;8 (in percent by weight) required for a high resistance to alkali solutions or that they do not satisfy additional requirements with respect to the portions of SiO.sub.2, Al.sub.2 O.sub.3 and ZrO.sub.2 (see Tables 1, 2 and 3) or of MgO, CaO, BaO and ZnO.
A typical example of a glass composition which is not optimal in the sense of the invention is the glass of the JENA.sup.er apparatus glass type of rounded composition in percent by weight on oxide basis) SiO.sub.2 75; B.sub.2 O.sub.3 10; Al.sub.2 O.sub.3 5; Na.sub.2 O+K.sub.2 O 5; CaO+BaO 5 which, in modified form, reaches a ratio of SiO.sub.2 :B.sub.2 O.sub.3 =8.4 (see Table 1, Example 2 as JENA.sup.er G 20) but does not contain ZrO.sub.2 and reaches only 80.5 percent by weight in the sum SiO.sub.2 +Al.sub.2 O.sub.3 (+ZrO.sub.2).
A further example of a resistance to alkali solution which is not optimal is given by the commercial glass SUPRAX.TM./8486.
SUPRAX.TM./8488 defines a modified SUPRAX.TM. which achieves an improved alkali resistance within LBK-Class 2 because of a ZrO.sub.2 admixture but which does likewise not correspond to LBK-Class 1.
However, the glass does not satisfy the conditions in accordance with the invention with reference to the base glass. For this reason, a decisive improvement of the alkali resistance cannot be obtained by introducing ZrO.sub.2.
Further examples of portions of MO (MgO, CaO, BaO, SrO, ZnO), which are too high, are shown in Table 3. When glasses have relatively high portions of MgO, CaO, BaO, SrO and ZnO (sum MO&gt;3 percent by weight), it is not possible to adjust the conditions of the invention with respect to proportions and ratios of the components for realizing a high resistance to alkali solutions. The MO-rich glasses furthermore have the disadvantage that they make possible only a relatively slight UV-transmission because of the inherent absorption of these oxides.
A great disadvantage of the discussed glasses is that they contain no lithium ions and therefore are not suitable to chemical tempering below the transformation temperature, that is, it is often not recognized nor is it considered that thermally temperable glasses must satisfy specific requirements with respect to the ratio of the thermal expansion coefficients above T.sub.g to below T.sub.g.
The thermal temperability of borosilicate glasses of high resistance to thermal shock or high thermal stress factors and thermal expansion coefficients .alpha..sub.20/300 =3.39-5.32.times.10.sup.-6 K.sup.-1 is discussed in U.S. Pat. No. 4,259,118 and it has been found that for this group of glasses, the ratio .alpha.'/.alpha. of the thermal expansion coefficients above T.sub.g (.alpha.') to below T.sub.g (.alpha.) should be between 4.1 and 9.4 in order to provide improved temperability and therefore mechanically stronger glasses. However, it is a disadvantage that these glasses contain a MO-portion which is too great and an Al.sub.2 O.sub.3 -portion which is too low so that the synthesis conditions to obtain a high alkali resistance are not given (see Table 4).
The significance of the thermal stress factor R is noted in German Patent 2,756,555 and is defined there as presented below: EQU R=.sigma..sub.B (1-.mu.)/.alpha.E
wherein:
R--thermal stress factor PA0 .sigma..sub.B --bending tensile strength (basic strength+pressure tempering) PA0 .mu.--poisson's ratio .alpha.--mean linear thermal expansion coefficient PA0 E--Young's modulus
As noted in the text of Salmang-Scholze entitled "Die physikalischen und chemischen Grundlagen der Keramik", Springer Verlag, (1968), pages 334 to 337, R is the first thermal stress factor which is applicable to large heat transfer numbers and gives the maximum temperature difference (.DELTA.T.sub.max) which a body can still sustain without being destroyed.
For partially heated glasses which are subjected to high thermal load, a large thermal stress factor is desired in accordance with the above-mentioned relationship. This thermal stress factor is to be influenced by large values of .sigma.B (for example, by applying a large pressure pretempering) or low thermal expansion coefficients.
In the literature, the significance of a high alkali resistance with a simultaneously high hydrolytic and acid stability for borosilicate glass 3.3 is often emphasized. In this connection, reference can be made to the publication entitled "SCHOTT Information", 56/91, Number 3, pages 18 to 20.
A high alkali resistance can, in some cases, also be desirable even though the direct application of the glass does not necessarily require this.
With the storage of conventional float glass panels of LBK-Class 2, for example, condensate forms. The formation of condensate leads to the alkali attack and therefore to the dissolution of the network so that an increased alkali resistance would be desirable. In this context, reference can be made to the paper of M. Feldmann entitled "Untersuchung korrodierter Floatglasoberflachen" 66, DGG-Tagung, Fulda, May 1992. From this observation, one can derive that floated borosilicate glass should have a very high alkali resistance.